The Regs do not specify this for pilots. It's just style points that most of us don't say it on center since there is good separation at Center altitudes. I have had US Center Controllers call me "Heavy" at altitude, even when it didn't match the exceptions in their Regs:

2-4-14. WORDS AND PHRASES
a. Use the words or phrases in radiotelephone and
interphone communication as contained in the P/CG
or, within areas where Controller Pilot Data Link
Communications (CPDLC) is in use, the phraseology
contained in the applicable CPDLC message set.
b. The word “heavy” shall be used as part of the
identification of heavy jet aircraft as follows:
TERMINAL. In all communications with or about
heavy jet aircraft.
EN ROUTE. The use of the word heavy may be
omitted except as follows:
1. In communications with a terminal facility
about heavy jet operations.
2. In communications with or about heavy jet
aircraft with regard to an airport where the en route
center is providing approach control service.
3. In communications with or about heavy jet
aircraft when the separation from a following aircraft
may become less than 5 miles by approved procedure.
4. When issuing traffic advisories.
EXAMPLE-
“United Fifty-Eight Heavy.”
NOTEMost
airlines will use the word “heavy” following the
company prefix and flight number when establishing
communications or when changing frequencies within a
terminal facility's area.
5. When in radio communications with “Air
Force One” or “Air Force Two,” do not add the heavy
designator to the call sign. State only the call sign “Air
Force One/Two” regardless of the type aircraft.